The isolated spinal cord of the frog will be used to study mechanisms of synaptic transmission and the factors that modulate them at central neurons. Emphasis will be placed on the role of sex hormones in affecting synaptic transmission and neuronal membrane properties in the neural control of reproductive behavior. The motor expression of sex behavior is stereotyped, especially as the "clasp reflex" during amplexus. Initially, histochemical studies will be done to identify the neurons involved in initiating this behavior. Labelled testosterone will be given over a period of days to the frog until clasp reflexes are regularly elicited and then the neurons which have taken up the label will be identified at all levels of the brain and spinal cord by autoradiography. If cervical spinal motoneurons take up the label, they will be compared to those which can be "back-filled" by horseradish peroxidase injection into the muscles of the forelimb involved in the clasp reflex. If motoneurons do not take up label, the projections to them from neurons at higher levels which are known to take up label, will be studied. Electrophysiological studies will be done in five groups of frogs. 1) normal males, not sexually active; 2) normal sexually active males; 3) castrated males; 4) castrated males to whom sex hormone has been given chronically in vivo; 5) normal males whose spinal cords will be isolated and sex hormone given in vitro. Extra- and intracellular recording techniques will be used to examine synaptic activity and membrane properties of spinal neurons of animals from the five groups described above. Patterns of spontaneous miniature epsps, spontaneous action potentials and elicited spikes and synaptic potentials will be analyzed; membrane resistance changes and ion species whose conductance across the neuronal membrane is influenced by the hormone will also be examined. The effects of the hormone on quantal transmitter release at the neuromuscular junctions of those muscles of the forelimb involved in amplexus will be measured. Finally, biochemical studies will be done to examine the possible roles of monoamines and cyclic nucleotides in the mediation of the neural effects of the hormones.